The present invention is generally directed to the control of a generator driven welding power supply, and more particularly directed toward a method and apparatus for electronically controlling the volt-ampere (V-A) output characteristics of a generator driven welding power supply.
Welding power supplies may be driven either by a DC generator or an AC generator (also called an alternator-rectifier). An AC generator generally includes, in addition to an alternator, a reactor followed by rectifiers to provide a DC output. AC driven welding power supplies are, generally, constant current type machines and the output volt-ampere characteristic of an AC driven welding power supply typically has a very steep slope. That is, the output current is nearly constant over a varying range of output voltage. Thus, welding power supplies that are driven by an AC reactor generator are particularly suited for welding applications requiring an approximately constant current output (such as flat plate welding). However, to significantly change the current output a different tap on the reactor must be provided (and selected by the user. This is both costly and inconvenient.
AC reactor generators are less suited for some applications. For example, when welding pipes the welder often needs to control the temperature of the welding puddle. For example, the welding puddle is cooled to prevent dripping, or heated to prevent the electrode from sticking in the welding puddle when holding a tight arc. This control is typically performed by varying the distance of the electrode from the workpiece. The nearly constant current characteristic of an AC machine does not easily allow such control because current does not vary with arc length (and arc voltage).
Also, it is useful in pipe welding applications to provide a set of output V-A curves having a single open circuit output voltage, then having a sloping characteristic down to a predetermined voltage level, then having a vertical characteristic (i.e., constant current) over a range of output voltages, and then again having a sloping characteristic to provide high output current for short circuit (i.e., tight arc) conditions. Accordingly, a generator driven welding power supply having output V-A curves with multiple breakpoints or multiple slopes is desirable. Preferably, the shape of the output curves can be electronically altered and optimized for other welding applications, such as TIG, GMAW, and FCAW.
It is important to be able to control the output in welding power supplies using feedback. One AC generator welding power supply includes a field control current feedback. That is, the field current was compared to a field current set point, and adjusted in response to deviations therefrom. However, this did not necessarily mean that the output weld current was maintained at the desired level. Another feedback control for constant voltage operation was providing the output voltage as feedback to the field current control. In this manner the field current could be adjusted to maintain a constant output voltage. However, neither scheme provides for output current control. Accordingly, it is desirable to provide a generator driven welding power supply that utilizes both output current feedback and output voltage feedback to electronically control the field current.
DC generators, in contrast, have a V-A output characteristic that is linear in nature. That is, the slope of the output V-A curve is constant such that as output voltage decreases, output current increases, i.e., DC machines provide a "droop" in the V-A curve. Thus, a welder using a DC driven welding power supply can easily control the temperature of the welding puddle by varying the distance of the electrode from the workpiece. When the electrode is pulled back, the arc (output) voltage increases while the output current decreases. When the electrode is in close proximity to the workpiece, the arc voltage decreases (short circuit condition) while the output current increases. The output characteristics of DC driven welding supplies are therefore more optimally suited for applications requiring more accurate control of the welding arc current (such as pipe welding).
Prior art DC generators may also include a series compound field winding. The compound field winding may add or subtract from the field to control the output of the generator. Multiple taps on the compound winding allow for range selection. This design is costly in that an additional field winding is required.
Welding power supplies driven by either DC or AC generators have disadvantages. For example, DC generators are generally more expense and difficult to manufacture than AC generators. In addition, both DC and AC driven welding power supplies generally include costly components, such as a rheostat and a range switch for selecting the maximum output current and slope of the V-A curve, respectively. The AC driven power supply also includes a costly tapped reactor. Thus, it is desirable to provide a method of controlling the output V-A characteristic of welding power supplies that eliminates the disadvantages associated with manufacturing DC generators, as well as the need for a rheostat, range switch, and tapped reactor.
In addition to controlling the output V-A characteristic, there is a need for a welding power having a fast transient response. A fast transient response is desirable because the welding puddle has a short thermal time constant and will thus chill if the output current of the welding power supply does not respond quickly to changes in arc length. A chilled puddle will cause the electrode to stick, creating undesirable inclusions in the weld. The transient response of a DC driven welding supply is quicker than the transient response of an AC reactor generator, providing the user with fast changes in output current when varying the distance of the electrode from the workpiece. Thus, DC driven welding supplies are better suited for applications requiring quicker control of welding arc current. A need therefore exists for a welding power supply combining the V-A characteristic and transient response of a DC driven supply with the cost-savings attainable with an AC driven supply.
Another disadvantage of existing welding power supplies supplied by either a DC or AC generator is that neither method provides optimal control of the open circuit output voltage. An AC driven welding power supply has no inherent open circuit output voltage. A DC driven supply has an open circuit output voltage that drifts as the operating temperature of the supply changes, and that varies widely as the rheostat is adjusted from a minimum to maximum setting. An uncontrolled or variable open circuit output voltage has the undesirable feature of providing an inconsistent starting characteristic. For example, if the open circuit voltage is too low the arc may be difficult to start. A need thus exists for a welding power supply capable of providing and maintaining a stable open circuit output voltage.
Many generator driven welding power supplies provide a 110 volt auxiliary power source. Such power sources are used for hand tools, lights, etc. Because the devices powered by the auxiliary power are designed to operate using line current, it is desirable to provide a "flat" V-A curve, i.e., a constant voltage, regardless of the current draw. This is in direct contrast to the output desirable for many welding application. Accordingly, it is desirable to provide a generator driven welding power supply that provides a droop in the welding output, but a flat auxiliary output.
It is further desirable to provide a welding power supply having an output V-A characteristic that can be optimally shaped and controlled for a variety of welding applications, including both constant current applications (e.g., SMAW pipe welding, stick welding) or constant voltage applications (e.g., MIG, flux core). Such a design would, optimally, replace the rheostat, range switch, and tapped reactor of existing designs with a printed circuit board and a single control knob and an inductor. In addition, it would preferably include provisions for a fast transient response and a constant, stable open circuit output voltage. Further, the control method would hopefully be able to be used in a variety of welding power supplies, including DC generators, single-phase AC generators, or a multiple-phase AC generators driven by an engine.